Frequency dependent hydraulic dampers from prior art work on the principle of having a frequency dependent bypass channel to a main piston. These dampers still require a conventional main piston assembly to set the low frequency damping characteristics, and require a significant (˜10 mm2) bypass flow channel to be modified into the rod's piston post. Examples of such prior art dampers are disclosed, for instance in documents WO03040586, WO2013051934, WO2015130544, WO2015030884.
There are three main problems of these dampers:
1) Reduced safety margin due to removal of material from a crucial structural location, the piston post, with possible fatal consequences in case of a strut suspension under side load.
2) Frequency dependent effect less effective at higher damper speeds due to restriction posed by limited flow channel in piston post.
3) Reduced damper travel leading to more end stop impacts which speed up damage to vehicle body and reduce comfort.
The damper disclosed in US2015276005 partially solves problems 3) and 2), but does not solve problem 1) and has the additional problem of: 4) Only being frequency dependent in one direction.